Lyme disease, the leading arthropod borne disease in N. America and Europe, is caused by the spirochetes Borrelia burgdorferi, B. garinii and B. afzelii. As the endemic regions and incidence of Lyme disease increase, there is a pressing need to advance our understanding of molecular mechanisms of pathogenesis of its causative agents. Outer surface protein C (OspC) is an immmunodominant antigen that has been implicated as an important virulence factor. The precise role(s) of OspC in pathogenesis are not clearly defined and remains the subject of intensive investigation and debate. It has been proposed that OspC plays a critical role in the transmission process and or in the establishment of infection in the mammalian host. With the development of genetic manipulation methodologies for the Borrelia and the determination of high resolution crystal structures for OspC, it is now possible to utilize and exploit this information to identify the determinants of OspC that contribute to virulence and pathogenesis. In this application, unique approaches are proposed for identifying mammalian produced ligands for OspC, mapping its functional domains and determining which specific steps in the infectious cycle OspC participates in.le.